The present invention relates to a sense amplifier for ascertaining the content of a memory device which is to be read. The content of the memory device is ascertained by comparing a current or voltage which is supplied to the sense amplifier as an input signal characterizing the content of the memory device and a threshold value.
Such sense amplifiers are used, for example, for reading data stored in a semiconductor memory (for example in an EEPROM), more precisely in the memory cells thereof.
When a memory cell in a semiconductor memory is read, a current or voltage is produced whose size is dependent on the content of the memory cell which is to be read, as is known. The sense amplifier compares this current or this voltage with a threshold value and determines the content of the memory cell which is to be read on the basis of the result of this comparison.
Experience shows that, with higher or lower frequency, sense amplifiers ascertain the content of a memory device which is to be read, such as a memory cell in a semiconductor memory, incorrectly.
Such errors can be prevented, in some cases, if the threshold value is stipulated such that it lies exactly in the center of the currents or voltages which are established on the basis of the content of a memory cell when the memory cell is read. However, even this does not provide a reliable way of ensuring that the sense amplifier always ascertains the content of the memory cell correctly.
The object of the present invention is to provide a sense amplifier which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and in which the risk that the content of a memory device is determined incorrectly can be reduced to a minimum.
With the above and other objects in view there is provided, in accordance with the invention, a sense amplifier for ascertaining a memory content of a memory device, comprising:
an input receiving an input signal characterizing a content of a memory device in a first memory state or profile and in a second memory state or profile, and an output carrying an output signal representing the content of the memory device;
a comparator device for comparing a current or a voltage of the input signal with a threshold value and determining therefrom a memory content of the memory device;
wherein the threshold value is defined as a first threshold value to be exceeded or undershot by the input signal in order for the output signal representing the content of the memory device to change from the first state or profile to the second state or profile, and as a second threshold value, higher or lower than the first threshold value, to be exceeded or undershot by the input signal in order for the output signal to change from the second state or profile to the first state or profile.
In other words, the sense amplifier according to the invention is distinguished in that the threshold value which the sense amplifier""s input signal characterizing the content of the memory device needs to exceed or fall below in order for an output signal from the sense amplifier, which represents the content of the memory device, to change from a state or profile which represents a first memory device content to a state or profile which represents a second memory device content is higher or lower than the threshold value which the sense amplifier""s input signal characterizing the content of the memory device needs to exceed or fall below in order for the output signal from the sense amplifier to change from a state or profile which represents the second memory device content to a state or profile which represents the first memory device content.
This makes it possible to prevent the states which are established in the sense amplifier, and hence possibly also the output signal from the sense amplifier, from repeatedly changing while the memory device (for example a memory cell in a semiconductor memory) is being read.
The states established in the sense amplifier and the output signal from the sense amplifier change repeatedly, in particular, when switching operations taking place within or outside the sense amplifier cause changes in the supply voltage and/or produce interference signals which affect the operation of the sense amplifier and/or the size of the currents or voltages which are to be compared by said sense amplifier. This can cause the sense amplifier and/or the output signal from the sense amplifier to begin to oscillate to the full. This is particularly the case when the various currents or voltages produced when a memory device is read are relatively close to one another.
A suitable choice of the various threshold values which the signal which is input into the sense amplifier needs to exceed or fall below in order for the output signal from the sense amplifier to jump from the low level to the high level or from the high level to the low level makes it possible to prevent the aforementioned interference from being able to cause any change in the states which are established in the sense amplifier and/or in the output signal.
Since sense amplifiers, like any other circuits, are generally designed from the outset such that the interfering influence of switching operations or other events on the currents and voltages produced within and outside the circuit is relatively slight, a relatively slight difference between the various threshold values is sufficient to prevent the effects which are based on such interference. This ensures that there is a sufficiently great distance between the threshold values and the currents or voltages which are to be compared with these threshold values, so that the use of the various threshold values has no negative influence on ascertainment of the actual memory cell content.
The sense amplifier according to the invention thus makes it possible to reduce the risk of the content of a memory device being determined incorrectly to a minimum.
In accordance with an added feature of the invention, the threshold value used for the comparison is selected or stipulated on the basis of the respective state or profile of the output signal from the sense amplifier.
In accordance with an additional feature of the invention, the output signal indicates whether the input signal characterizing the content of the memory device lies below or above the threshold value used before the output signal last changed over.
In accordance with another feature of the invention, the threshold value used when the output signal represents the first memory device content is closer to the input signal established when the content of the memory device is the second memory device content than the threshold value used when the output signal represents the second memory device content.
In accordance with a further feature of the invention, the memory device is a memory cell in a semiconductor memory.
In accordance with again an added feature of the invention, there is provided a differential amplifier having a first input receiving a voltage characterizing the content of the memory device and a second input receiving a reference voltage.
In accordance with again an additional feature of the invention, the differential amplifier is configured to compare the voltages at the first and second inputs, but the comparison is specifically perturbed by an offset voltage. The offset signal, which is generated in the differential amplifier, corresponds to an offset voltage or acts as an offset voltage. Preferably, the offset voltage is a variable voltage. The offset voltage is variable in magnitude and/or in its arithmetic sign.
In accordance with again an additional feature of the invention, the magnitude and/or arithmetic sign of the offset voltage is changed based on an output signal of the differential amplifier.
In accordance with again another feature of the invention, an output signal of the differential amplifier indicates whether the voltage characterizing the content of the memory device is higher than a first threshold voltage or lower than a second threshold voltage, which is lower than the first threshold voltage. Preferably, the first threshold voltage is higher than the reference voltage. Similarly, the second threshold voltage is lower than the reference voltage.
In accordance with again a further feature of the invention, a difference between the first threshold voltage and the reference voltage and/or a difference between the second threshold voltage and the reference voltage is/are dependent on the offset voltage produced in the differential amplifier that corresponds to an offset voltage or acting as an offset voltage.
In accordance with yet an added feature of the invention, the offset voltage is produced by impressing an additional current into the differential amplifier. In a preferred embodiment, a current control stage is provided to output the additional current based on the output signal from the sense amplifier.
In accordance with yet an additional feature of the invention, the current control stage impresses the additional current at a point of the differential amplifier which is dependent on the output signal of the sense amplifier.
In accordance with yet another feature of the invention, the voltage produced in the differential amplifier and corresponding to an offset voltage or acting as an offset voltage is a voltage dropping across a resistance (a resistor or a circuit component acting as a resistor) in the differential amplifier, with a flow of current through the resistance that produces the voltage drop dependent on the additional current impressed into the differential amplifier. A direction of the current flowing through the resistance is dependent on a point at which the current is impressed into the differential amplifier.
In accordance with yet again an added feature of the invention, one of the voltages supplied to the differential amplifier via the inputs is varied on a basis of the output signal of the differential amplifier.
In accordance with a concomitant feature of the invention, a supply voltage for the differential amplifier is varied on the basis of the output signal of the differential amplifier.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a sense amplifier, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.